7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Simulations of energetic-particle driven Alfvén eigenmodes inmagnetically confined plasmas
Y. Todo1
1National Institute for Fusion Science, National Institutes of Natural Sciencese-mail: todo@nifs.ac.jp
In burning plasmas, which is expected to be realized in
ITER, energetic alpha particles born fromdeuterium-tritium (D-T) fusion reaction will play anessential role in heating the fuel plasma through particle
collisions. Confinement of energetic alpha particles iscrucial for the sustainment of high temperature neededfor fusion reaction, which is higher than 10 keV. In thepresent magnetic confinement experiments on tokamak
and stellarator/heliotron devices, auxiliary heating suchas
neutralbeam
injection(NBI)
andion-cyclotron-range-of-frequency (ICRF) wave heating
generate fast ions. Confinement of the fast ions is also
important for the present experiments.
Alfvén eigenmodes (AEs) are magnetohydrodynamic(MHD) oscillations in magnetically confined plasmas
and excited by energetic particles (EPs) such as fast ionsgenerated by NBI and ICRF heating. EPs are circulating
inside the plasma along the magnetic field line and candestabilize AEs through resonant interaction. This is akind of inverse Landau damping with MHD waves in
3-dimensional magnetically confined plasmas withcomplicated particle orbits and non-uniform spatialdistribution. The spatial gradient of the EP distribution
can drive AEs as well as the distribution gradient invelocity space. What is important is that this problem is awave-particle interaction in an open system with source(fusion reaction, NBI, and ICRF) and sink (losses). This
results in various types of time evolution, steady
amplitude, frequency chirping, and recurrent bursts.
Validation studies of kinetic-MHD hybrid simulation
code MEGA were conducted for DIII-D tokamak
experiments where significant flattening of the fast-ion
distribution associated with many AEs with low
amplitudes were observed [1-3]. In the validation studies,realistic NBI as well as particle collisions and losses
were implemented in the MEGA code. A novel method
was developed to realize the simulation of theslowing-down time scale of fast ions [1]. The
significantly flattened fast-ion pressure profile observedin the experiment was reproduced with the MEGA
simulation [2]. In addition, the electron temperaturefluctuation profiles which are brought about by the AEs
are also well reproduced with the MEGA simulation forfrequency, spatial profile, and amplitude. These results
indicate that the MEGA code is a powerful tool for theunderstanding and the prediction of EP driven AEs andthe EP transport.
Frequency chirping of EP driven instabilities isobserved in fusion plasmas as well as in space plasmas.

For example, recurrent bursts of EP driven geodesic
acoustic modes (EGAMs) were observed with upward
frequency chirping in the Large Helical Device (LHD)[4]. The MEGA code was applied to an LHD experiment
where a secondary EGAM is suddenly excited during theupward frequency chirping of the primary EGAM [5].
The frequency of the secondary EGAM is just a half ofthat of the primary EGAM. This interesting phenomenon
was reproduced with the MEGA simulation. It was foundthat the secondary EGAM is excited by a fractional
resonance with the EPs that are resonating with theprimary EGAM.
Bursting evolution of AEs were observed in many
tokamaks and stellarators. The MEGA code was applied
to examine the distribution formation process in thecollisional slowing-down time scale of fast ions forvarious NBI power and collisional slowing-down time[6,7]. It was found that the intermittency of AEs rises
with increasing NBI power and increasing slowing-downtime. Figure 1 shows that with increasing
volume-averaged classical fast ion pressure, the fast ion
confinement degrades monotonically due to the transport
by the AEs [6].
References[1] Y. Todo et al., Nucl. Fusion 54 (2014) 104012[2] Y. Todo et al., Nucl. Fusion 55 (2015) 073020[3] Y. Todo et al., Nucl. Fusion 56 (2016) 112008[4] T. Ido et al., Phys. Rev. Lett. 116 (2016) 015002[5] H. Wang et al., Phys. Rev. Lett. 120 (2018) 175001[6] Y. Todo, New J. Phys. 18 (2016) 115005[7] Y. Todo, Nucl. Fusion 59 (2019) 096048

Figure 1 Volume-averaged fast ion beta versus
volume-averaged classical fast ion beta. Fast ion
confinement degrades with increasing classical fast ionbeta [6].